The polyamide resin is being widely used for usage such as film, sheet, packaging bag, engineering plastic and fiber, because of its excellent physical and mechanical properties.
In such usage, aliphatic polyamides such as nylon 6 and nylon 66 have been heretofore predominantly used. However, the aliphatic polyamide in general has drawbacks that a large dimensional change occurs between the water absorption moisture absorption time and the drying time and since this is aliphatic, the elastic modulus is small and the softness is excessively high. Therefore, a polyamide resin having higher performance is being demanded. Under these circumstances, an aromatic dicarboxylic acid such as TPA (terephthalic acid) and IPA (isophthalic acid) is copolymerized with a conventional aliphatic polyamide so as to attain high performance of the polyamide resin. For example, JP 59-155426 A and JP 62-156130 A disclose a polyamide resin in which TPA or IPA is copolymerized.
However, the introduction of an aromatic ring into the polyamide skeleton generally leads to elevation of melting point or melt viscosity, and the production of polyamide is confined to severer conditions such as temperature condition and encounters more acceleration in the production of gelled product, thermal degradation or deterioration due to thermal decomposition reaction. The gelled product may deposit in the polymerization rector to require more frequent cleaning operations or mingle into the resin, or the thermal degradation may bring about reduction in the physical properties, as a result, a high-quality polyamide resin cannot be obtained.
These are mainly attributable to the long-term residence of polyamide resin in a high-temperature condition, and various production methods have been proposed with an attempt to solve these problems. For example, according to the production methods disclosed in JP 60-206828 A, JP 2-187427 A and JP 8-170895 A, an initial condensate is once taken out as a prepolymer so as to avoid the long-term residence under high temperature, and this polymer is solid phase-polymerized at a temperature lower than the melting point of polymer, whereby the thermal decomposition degradation is inhibited. However, these methods all are a batch-system production method and are undesirable in view of production efficiency and also disadvantageous in that the quality readily differs among batches.
As for the polyamide, a large number of aromatic-containing polyamides are also known, where, for example, reduction of water absorption and elevation of elastic modulus are realized by using an aromatic diamine such as p-xylylenediamine (PXD) or m-xylylenediamine (MXD) as a raw material.
The raw materials used in the production of a polyamide are generally a diamine and a dicarboxylic acid as in the production of 6,6-nylon. In this case, for elevating the polymerization degree to a level of allowing for use as a product, it is important to control the mol balance between the diamine component and the dicarboxylic acid component. This problem is generally overcome by employing a method of charging two components of diamine component and dicarboxylic acid component in the form of an aqueous solution, and adjusting the pH to form a salt of amine and carboxylic acid. However, the salt forming method is disadvantageous in that since a large amount of water content must be removed so as to allow the polymerization reaction to proceed, a large quantity of heat is necessary as compared with the amount of production and moreover, the apparatus becomes large. Furthermore, when continuous production is intended, the pH adjustment performed every each batch takes time and therefore, the efficiency is low.
In order to solve these problems in the polymerization method using an aqueous solution of salt, a method of performing continuous polymerization of polyamide without using water as a solvent has been proposed.
For example, JP 10-509760 T employs a method of supplying a dicarboxylic acid excess component in the melted state to a multistage reactor, and adding the lacking diamine component to the reactor. However, in this method, the addition of diamine and the polymerization reaction must be performed in parallel in the polymerization reactor and therefore, the apparatus structure becomes special and complicated.
JP 2001-200052 A discloses a continuous production method of a polyamide, which comprises a step of continuously supplying a slurry comprising a xylylenediamine-containing diamine and a dicarboxylic acid to a ventless twin-screw extruder and heating it to allow for proceeding of amidation reaction, and a step of elevating the polymerization degree of polyamide in a single-screw extruder with a vent while separating and removing the condensed water produced by the amidation reaction. According to this patent publication, a slurry solution of diamine and dicarboxylic acid is prepared by a batch system at a low temperature of 80° C. or less and then the polymerization reaction is initiated. In this method, the problem of large apparatus in the aqueous solution polymerization is overcome, but for preparing a slurry solution without causing an amidation reaction rich in reactivity, the temperature and moisture percentage must be strictly controlled and moreover, the preparation of a homogeneous slurry solution takes time, giving rise to a problem in the productivity. The molecular weight of the polyamide obtained in Examples of this patent publication is low and approximately from 3,000 to 5,000.
JP 2002-516366 T discloses a continuous production method of nylon 66. According to this patent publication, a fused dicarboxylic acid and a fused diamine are mixed in equimolar amounts by using a raw material weighing system to produce a fused reaction mixture, the reaction mixture is passed to a non-ventilative reaction apparatus (static in-line mixer) to form a first product flow containing polyamide and condensed water, the first product flow is supplied to a ventilated tank-type reaction vessel to remove the condensed water and form a second product flow containing polyamide, the second product flow is measured by near infrared spectroscopy to determine relative amounts of amine end group and carboxylic acid end group and based on the values obtained, the dicarboxylic acid weighing system and/or diamine weighing system are controlled.
JP 2002-516365 T discloses a control system for controlling the mol balance between fused dicarboxylic acid and fused diamine, where the balance between carboxylic acid end group and amine end group in the polymerization mixture is detected by using a near infrared spectrometer and based on the detection results, the mass flow rate of at least one of the fused dicarboxylic acid and the fused diamine is adjusted.
However, when the mol balance between dicarboxylic acid and diamine is controlled by the feed back from the downstream polymerization mixture to the upstream raw material supply part, time lag is caused and this is not preferred in respect that the mol balance is difficult to exactly control at all times. Also, such a control system is disadvantageously complicated and costs high.
In order to express properties suitable for respective uses described above, a polyamide having a desired polymerization degree is necessary. Generally, in producing a polyamide, the polymerization degree of product polyamide is determined by measuring its relative viscosity [RV]. The relative viscosity is one of the most important indices in confirming the quality of polyamide.
The polymerization degree has a relationship with, for example, reaction temperature, inner pressure (vacuum degree) of reactor, end group concentration of polymer (addition of end group adjusting agent such as acid anhydride), and moisture percentage in the gas phase at the interface of fused polymer during reaction and therefore, a method of adjusting the polymerization degree by changing any one of these conditions is generally employed.
However, when the polymerization degree is intended to adjust by using only one of those conditions, various status changes are generated other than the polymerization degree and this may adversely affect the quality of polyamide produced. For example, when only the degree of vacuum is adjusted to a high vacuum degree with an attempt to obtain a high polymerization degree, the residence amount of polymer in the polymerization reactor is varied due to change in the holdup brought about by bubbling and generation of contamination, change in the residence time or the like is caused, as a result, a polyamide having an intended quality can be hardly obtained.
Also, when only the end group concentration of polymer is adjusted by the addition of an end group adjusting agent with an attempt to obtain a desired polymerization degree, the end group adjusting agent disadvantageously remains in the product polyamide in a larger amount along with increase in the amount added of the end group adjusting agent.